1. Field of the Invention
The present invention relates to an electronic control unit having a microprocessor built-in and, more particularly, to improvements of an electronic control unit to which a monitoring control circuit is added in order to improve safety in control.
2. Description of the Related Art
To monitor operation of a microprocessor with a watchdog signal, or to check contents in a program memory during the operation, several attempts have been proposed. For example, the Japanese Patent Publication (unexamined) No. 97042/1993 proposed an art in which a main CPU has function to monitor cycles of a watchdog pulse signal that is outputted from a sub CPU, and detect errors of the sub CPU based on the results thereof; and likewise the sub CPU has function to monitor cycles of a watchdog pulse signal that is outputted from the main CPU, and detect errors of the main CPU based on the results thereof. Thus, the main CPU and the sub CPU detect errors with respect to each other.
The Japanese Patent Publication (unexamined) No. 81222/1993 proposed run-away monitoring means in a system consisting of two CPUs of a main CPU and a sub CPU. In this run-away monitoring means, operation of the main CPU is monitored by a watchdog timer, which is provided outside of the main CPU, and operation of the sub CPU is monitored by the main CPU.
The Japanese Patent Publication (unexamined) No. 227402/2001 proposed a further art in which contents of a ROM memory storing a control program, discrimination data or the like therein is subject to time division to carry out a sum check thereof in accordance with a processing load of a microprocessor. Thus, presence or absence of any error in the control program is detected.
In association with this invention, the Japanese Patent Publication (unexamined) No. 162814/1997 disclosed an art in which a communication-monitoring device includes: a communication control section that controls a data communication and outputs an error notification signal upon detecting a communication error; an error counter that performs up-count in accordance with the input of an error notification signal; and annunciation section that announces the communication error responsive to the fact that a count value of the error counter has reached a positive predetermined value. This communication-monitoring device further includes counter subtraction means for counting down a count value of the error counter responsive to the fact that the control of data communication is normally carried out in the communication control section.
Further, the Japanese Patent Publication (unexamined) No. 97042/1993 and the Japanese Patent Publication (unexamined) No. 81222/1993 disclosed an art mainly intending to monitor run-away of a main CPU or a sub CPU, which carries out no monitoring as to contents of the control by the CPU.
Furthermore, the Japanese Patent Publication (unexamined) No. 227402/2001 discloses an art in which sum check of contents of a program memory relating to the control content is conducted, so that improvement in reliability of the control is expected. This is, however, a self-diagnosis function to the end. Thus, there remains a disadvantage in that the suspected one checks its suspected region and it is not any diagnosis of the very control operation.
In addition, the Japanese Patent Publication (unexamined) No. 162814/1997 intends to provide countermeasure after occurrence of any communication error, which is not intended to detect any error in control operation.